feat(gateway): add e2e test for cached input cost validation (#904)

Extended e2e chat tests to validate cached input cost functionality.
Added logic to verify that cached input cost is greater than zero when
supported.

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## Summary by CodeRabbit

- New Features
- Added prompt caching support for Claude/Anthropic models, including
ephemeral cache markers and updated headers.
- Enhanced token usage and cost calculations to account for cached input
tokens; UI now shows cached tokens and cached input cost in logs.

- Tests
- Introduced end-to-end tests validating caching behavior and cost
reporting across providers.

- Documentation
  - Added guidance on strict TypeScript usage in contributor docs.

- Chores
  - Optimized test scripts to skip building UI/docs during test runs.

- Refactor
- Removed debug logging and streamlined parsing paths without changing
behavior.
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feat(chats): enhance message schema to support images (#652)

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## Summary by CodeRabbit

* **New Features**
  * Chats now support image attachments; message text is optional.
* Playground displays and saves images from both streaming and
non‑streaming replies.

* **Bug Fixes**
  * Copy-to-clipboard works even when a message has no text.

* **Documentation**
* API reference pages reformatted for readability (no behavioral
changes).
* Changelog updated with “Try it now in the Playground” links for the
Gemini 2.5 Flash Image Preview.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Co-authored-by: Luca Steeb <[email protected]>

refactor(infra): use non-static builds for frontends (#444)

Co-authored-by: Cursor Agent <[email protected]>